1. Field of the Invention
The present invention relates to an electron microscope and the like and more particularly to a method of positioning the electron beam irradiation and a device used in such method which enables a very small area to be analyzed so as to easily confirm the position of the portion of a specimen which has been irradiated.
2. Description of the Prior Art
In the past, very small area analysis under an electron microscope was carried out together with a scanning image observation device (Hitachi technical data EM sheet No. 28). As a very slender specimen has been used lately, resolution has become insufficient.
Thus, in recent years, a method in which an object lens is used during strong excitation and an electron beam is converged in its front magnetic field and image formation and an enlargement is made in the back magnetic field under a specimen has been used.
In this method, an analysis is made in such a way that while a specimen is being observed on a fluorescent screen in the condition that an electron beam is expanded, a portion of the specimen to be analyzed is positioned around the center of the fluorescent screen, the shape of a portion to be analyzed is remembered. After the electron beam is slenderly converged, the electron beam is moved to a remembered portion by a deflecting device.
The spot size of the electron beam at this time is several nm. Even if the electron beam is expanded on the order of one million times, this is merely several mm and it is very difficult to know which portion of a specimen is being seen.
Since an electron lens is used during strong excitation, even when the portion to be analyzed has been confirmed beforehand, the electron beam does not necessarily converge at the same place due to an influence such as the saturation of a magnetic path. Furthermore, in a portion of a specimen off an optical axis, the position of the image moves due to the convergence of an electron beam, with the result that it is unclear whether or not the electron beam has reliably irradiated the portion of a specimen to be analyzed.
As another example of the prior art, that of Japanese Unexamined Patent Publication No. 1-112648 can be cited. In this patent the position of a specimen is stored in an image memory (and data of the slight movements of a specimen) and displayed, and the position of an electron-beam probe is overlapped-and displayed on a display device. The example of
Patent Unexamined Publication No. 63-213247 is one in which to make centering (the operation of aligning the center of a fluorescent screen with the center of electron beam) easy, A.C. waveforms are superimposed in either the X or Y directions. Japanese Patent Unexamined Publication No. 57-111938 is one in which focusing of a selected-field aperture decreased image is made easy. Japanese Patent Unexamined Publication No. 59-16256 relates to a specimen slight-movement device which converts the amount of a movement by an image shift (a device for electrically moving an image of a specimen) mechanically. U.S. Pat. No. 4,399,360 is one in which an image on a diaphragm provided in the lower part of a camera room is moved by applying a scanning signal voltage to a deflection electrode under a specimen, a signal is picked up by a detector under the diaphragm and the image of the specimen is displayed on a video display. U.S. Pat. No. 4,169,240 relates to auto focus of SEM and the like, in which high-frequency components of a video signal are picked up and the number of orders of a high-frequency filter is changed according to the number of frames used in auto focus. In all of the above examples of prior art, it is unclear whether or not the electron beam irradiates the portion of the specimen to be analyzed.